Lubricant system for use in powdered metals

ABSTRACT

The present invention provides a lubricant system in the pressing of powders to form a part. The lubricant system is a solid at ambient conditions. However, upon application of pressure during the pressing of the metal parts, the lubricant system forms a liquid phase along the walls of cavity in which the powder is being pressed.

[0001] This application claims priority to provisional applicationSerial No. 60/312,310 filed Aug. 14, 2001 and non-provisionalapplication Ser. No. 10/217,137 filed Aug. 12, 2002 both entitledLUBRICANT SYSTEM FOR USE IN POWDERED METALS.

FIELD OF INVENTION

[0002] The present invention concerns a lubricant system. Moreparticularly, the present invention concerns a lubricant system for usein powder metal compositions that are used in the production of metalparts.

BACKGROUND OF THE INVENTION

[0003] The use of powder metal compositions to produce metal products iswell-known in the prior art. Powder metals (powder metallurgy) iscommonly employed in applications wherein casting, forging or othermetal processing techniques is not cost effective. The fabrication ofparts using powder metals includes the steps of placing the metalcomposition in the cavity of a mold, pressing the composition to form agreen part, removing the green part from the cavity, and firing thegreen part.

[0004] During the pressing operation, it is generally important that alubricant be employed in order to facilitate the removal of the greenpart from the cavity and allow for the slippage of particles againsteach other so that forces are spread uniformly and density can be madeto be as uniform as possible in the cross-section of the pressed part.Generally, in the prior art there are two separate approaches to the useof lubricants. One approach is to apply the lubricant to the wall of themold cavity prior to adding the metal composition to the cavity, withthe metal composition having a relatively low level of lubricant. Thedownside to this approach is that it is time-consuming to apply auniform coating of a liquid lubricant to the cavity walls. The secondapproach is to incorporate a relatively higher level of lubricant intothe powder metal composition. However, the use of prior art lubricantsresults in several adverse effects. Specifically, such lubricants reducethe flow of the powder metal into the mold cavity thereby slowing thepressing operation. Lubricants can also detrimentally impact greendensity and result in the evolution of undesirable effluents duringpreheat and the sintering operation. Lubricants can also contribute tolow final density in parts, protracted furnace time, and the formationof cracks and blisters during firing.

[0005] The present invention provides a lubricant system that overcomesthe deficiencies of the prior art lubricant systems.

SUMMARY OF THE INVENTION

[0006] The present invention provides a new and improved lubricantsystem for use in powder metallurgy. The lubricant system is solid atambient conditions, but upon application of press pressure (formingpressure and stress) it transforms to a liquid phase. The lubricantsystem provides an excellent lubricant for use in powder metals for itresults in good flow of the powder composition, low loadingrequirements, shorter furnace times, ease of removal of the green partfrom the mold cavity and the formation of minimal effluents duringheating. In addition to the lubricant system, the present invention alsoprovides a metal mixture and method of using the same. The lubricantsystem of the present invention may also be used in connection with thepressing of ceramic powders.

[0007] The foregoing and other features of the invention are hereinaftermore fully described and particularly pointed out in the claims, thefollowing description setting forth in detail certain illustrativeembodiments of the invention, these being indicative, however, of but afew of the various ways in which the principles of the present inventionmay be employed.

DETAILED DESCRIPTION

[0008] The lubricant system of the present invention is a solid atambient conditions. Thus, the lubricant system is a solid when it ismixed with the powder metal. However, when pressure and stress isapplied to the lubricant system during the pressing of the metalcomposition, it must be capable of transforming at least in part to aliquid phase along the walls of the mold cavity.

[0009] There are various compositions that will work in accordance withthe invention. Preferably, the major components of the lubricant systemdisplay solubility with one another in the melt state. The melt pointsof the components of the lubricant system are depressed during melting,possibly forming some type of eutectic.

[0010] It is normal for a solid material to have an increase in meltingpoint as pressure is applied. There are other exceptions however. TheClapeyron-Clausius thermodynamic equation predicts that when the densityof a material in its solid state is less than the density of a substancein its liquid state, then the melting point of that substance willdecrease when pressure is increased. Applicant believes that thisequation may be used to predict material systems suitable for use in thepresent invention.

[0011] Applicant has found that materials that work flow under pressureand shear, flow better with increased shear and temperature, arepartially crystalline at room temperature. Transformation of a lubricantsystem to a liquid phase, preferably takes place at about 4 tons persquare inch at room temperature, a very low end of the working pressureof a press. The faster a press runs, the more shear is generated andtemperature due to particle-to-particle friction. Both of these actionsreduce the viscosity of a lubricant system. Normal press operationsimpart to a part a temperature of about 90° to about 140° F. Thus,applicant has found that a lubricant system that displays a viscosityrange of from about 1000 to about 6000 poise at a shear rate of 10001/second and a temperature of 100° F., performs well. By takingadvantage of the shear thinning properties of the lubricant system,non-dusting metal mixes can be made without the use of solvents, therebyalso resulting in metal mixes with reduced segregation of components,and the loss and segregation of minor ingredients. Also, preferablythere is a strong attraction by the lubricant system to the surface ofthe metal particles. Further, preferably the lubricant system cleanlybums during the firing or sintering of the green part, with no formationof undesired residual metals or undesired reduced metals. Additionally,the lubricant system of the present invention permits the operation ofpresses at much greater loads leading to improved green densities andparts free of defects such as blisters and delaminations.

[0012] One lubricant system that performs in accordance with therequirements of the present invention comprises a fatty acid material.The lubricant system may also include a wax, and in one preferredembodiment the lubricant system comprises a guanidine material. The waxmay be synthetic or natural. One preferred synthetic wax is an amidewax.

[0013] The fatty acid material of the present invention comprises acarboxylic acid derived from or contained in an animal or vegetable fator oil. Preferably, the fatty acid material comprises an unsaturatedfatty acid or a mixture of such acids, and salts thereof such as lithiumstearate. Examples of suitable unsaturated fatty acids include butyricacid, lauric acid, palmitic acid and stearic acid. More preferably, thefatty acid material comprises a mixture of lauric acid, palmitic acidand stearic acid. The fatty acid may also comprise a fatty acid estersuch as, for example, glycerol monostearate or butyl stearate.

[0014] In one embodiment, the guanidine material is a reaction productof guanidine and an acid selected from a fatty acid, an organic acid, ora stronger acid. The guanidine material is a reaction product which maybe an amide or actually may be more in the nature of a hydrated salt.For example, according to the CRC Handbook of Chemistry and Physics,74th Ed., guanidine acetate has the formula (H₂N)2,C═NH.CH₃COOH, ratherthan an amide-type formula such as H₂N—C═NH(NH)COCH₃, as would beexpected for an amide. This is due to the fact that guanidine is a verystrong base, and is much more likely to simply abstract a proton from arelatively weak organic acid, rather than react with the organic acid ina “standard” amidization reaction forming an amide with concomitant lossof H₂O. However, in some cases, the reaction of guanidine and the acidmay yield an amide in the “standard” manner. For this reason, theguanidine material of the present invention will be referred to hereinas the reaction product of guanidine and an acid. The term “reactionproduct of guanidine and an acid” includes both of the above-describedforms of the product of a reaction between or mixture of guanidine andan acid, and mixtures of these forms or other possible forms.

[0015] The particular acid used to make the reaction product ofguanidine and an acid is selected based upon obtaining desired effectswhen mixed with other compounds. In one embodiment, the guanidinematerial is guanidine stearate. In one embodiment, the guanidinematerial includes guanidine ethyl-hexonate. In other embodiments, theguanidine material may be the reaction product of guanidine and otheracids. The many acids which may be reacted with the guanidine to formthe reaction product of guanidine and an acid are described in detailhereafter.

[0016] According to the present invention, the guanidine material mayinclude the reaction product of guanidine and organic acids in the C₁₂to C₂₂ range. Thus, for example the reaction product of guanidine andoleic acid (C₁₇H₃₃CO₂H) would be suitable. Other suitable acids includesuch saturated fatty acids as (common names in parentheses) dodecanoic(lauric) acid, tridecanoic (tridecylic) acid, tetradecanoic (myristic)acid, pentadecanoic (pentadecylic) acid, hexadecanoic (palmitic) acid,heptadecanoic (margaric) acid, octadecanoic (stearic) acid, eicosanoic(arachidic) acid, 3,7,11,15-tetramethylhexadecanoic (phytanic) acid,monounsaturated, diunsaturated, triunsaturated and tetraunsaturatedanalogs of the foregoing saturated fatty acids. Additional organic acidsinclude acids such as ethylhexanoic acid (C₇H₁₅CO₂H), hexanoic acid,heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, anddodecanoic acid. Branched-chain carboxylic acids in the C₆ to C₁₂ rangemay also be used.

[0017] According to the present invention, the reaction product ofguanidine and stronger acids such as sulfonates, phthalates, benzoates,phosphates and phenbls may be used. For example, the reaction product ofguanidine and an acid such as benzenesulfonic acid may be used. As analternative, intermediate acids may be selected for reaction withguanidine. Alternatively, the guanidine material used in the lubricantcomposition may be the reaction product of guanidine and a weaker acidsuch as benzoic acid.

[0018] In a preferred embodiment, the guanidine material comprises amixture of guanidine stearate and guanidine ethyl-hexonate.

[0019] Synthetic waxes may also be used in the lubricant system of thepresent invention. Examples of such synthetic waxes include petroleumwaxes such as, for example, microcrystalline waxes and ethylene waxes,including ethylene copolymer waxes.

[0020] A preferred amide wax for use in the present invention is anethylenebisamide wax. The ethylenebisamide wax is a wax formed by theamidization reaction of ethylene diamine and a fatty acid. The fattyacid may be in the range from C₁₂ to C₂₂, but is usually made fromstearic acid, the saturated C₁₈ fatty acid. Thus, in one preferredembodiment, the ethylenebisamide wax is ethylenebisstearamide wax.Ethylenebisstearamide has a discrete melting point of about 142° C. Theethylenebisamide wax preferably has a discrete melting point in therange from about 120° C. to about 160° C.

[0021] In other embodiments of the lubricant composition, otherethylenebisamides include the bisamides formed from the fatty acidsranging from C₁₂ to C₃₀. Illustrative of these acids are lauric acid,palmitic acid, oleic acid, linoleic acid, linolenic acid, oleostearicacid, stearic acid, myristic acid and undecalinic acid. Unsaturatedforms of these fatty acids may also be used.

[0022] In one embodiment, the ethylenebisstearamide is ACRAWAX® C,available from LONZA, Inc. ACRAWAX® C has a discrete melt temperature of142° C.

[0023] Any of a variety of metal powders may be used in the presentinvention. Such metal powders include metallic (including alloys), metaloxide and intermetallic powders. Examples of common commercial metalpowders include steel powders, stainless steel powders, nickel, copperand brass. In addition to metal powders, applicant believes that thelubricant system of the present invention is suitable for use inconjunction with ceramic powders. Applicant hereby incorporates byreference U.S. Pat. No. 6,093,761, especially for its teachings relativeto inorganic powders. The powders of '761 may be employed in conjunctionwith the present lubricant system.

[0024] In one embodiment, the lubricant system comprises by weight fromabout 5% to about 35% fatty acid and from about 0.5% to about 50%guanidine material. In another embodiment, the lubricant system includesby weight from about 30% to about 65% amide wax. In one preferredembodiment the lubricant system comprises by weight from about 5% toabout 15% by weight lauric acid, from about 5% to about 15% by weightstearic acid, from about 40% to about 60% amide wax and from about 25%to about 40% guanidine material. Preferably, the guanidine materialcomprises a mixture of guanidine stearate and guanidine ethyl-hexonate.In one embodiment the guanidine stearate comprises by weight 10% toabout 25% of the lubricant system, and the guanidine ethyl-hexonatecomprises about 10% to about 20% of the lubricant system.

[0025] In another embodiment the lubricant comprises by weight fromabout 15% to about 30% ethylene wax. In another embodiment the lubricantsystem may comprise by weight from about 30% to about 70% amide wax andfrom about 5% to about 20% ethylene wax and up to about 30% fatty acidmaterial. In yet another embodiment, the lubricant system may compriseby weight from about 30% to about 70% of a fatty acid metal salt and upto about 35% fatty acid material, and up to about 30% of an ethylenewax.

[0026] The lubricant system is produced by first pregrinding the fattyacids to a particle size of about 100 mesh. A Waring Blender or othertype of grinding apparatus may be used. The fatty acids are then mixedwith the remaining components. Preferably, a double cone mixing step isused followed by melt mixing in an extruder. For small batches oflubricant, it is possible to simply do a batch melt of a homogeneousmixture. Finally, the melt mixed product is preferably cryogenicallyground to provide a particle size of about 10 to about 25μ microns.

[0027] Applicant has found that the rate of cooling of the lubricantmelt affects the viscosity of the resultant material, and thus coolingrate can be utilized to control and obtain the desired viscosity of thelubricant system.

[0028] The lubricant system may be mixed with the powders that are to beprocessed using conventional techniques. For example, a double conemixer may be used to mix the powder and the lubricant either in aready-to-use mix or a master (concentrated) mix. Also, the lubricantsystem may be used in various methods for forming a part includinginjection molding and standard pressing operations. Such methodsgenerally include the steps of placing the powder composition in a moldhaving a cavity, applying pressure to the composition to form a greenpart, removing the green part from the mold, and sintering the part toform a finished part. During the pressing operation, a portion of thelubricant system converts from a solid to a liquid phase. The. sinteringstep is conventional, and it is common to employ a preheat step whereina majority of the lubricant system is burned out of the green part.However, use of the present lubricant system results in significantlyless furnace time, which is a major cost saving feature for theend-user. Of course, the exact heating profile is primarily a functionof the composition of the powders being processed, loading density andspeed of processing.

[0029] When mixed with the metal powders, the concentration of thelubricant to the entire powder system is preferably in the range ofabout 0.10 to about 0.5 percent by weight. This loading is significantlyless than prior art lubricants wherein loadings of about 0.5-1.5% byweight of the total powder system are typical. Thus, the lubricantsystem of the present invention offers a significant reduction(generally 50% or greater reduction) in the amount of lubricant usedcompared to traditionally used lubricants due to the efficiency oflubrication from the transformation from a solid to a liquid.

[0030] Because less lubricant is utilized, green density increases dueto less volume of lubricant and due to particle-to-particlerearrangement caused by slippage at low pressure. Final part propertiesimprove as green density improves (e.g., sintered density, strength,hardness, greater uniformity and fewer defects). By use of the lubricantsystem of the present invention, excellent green densities are achievedwithout the use of special equipment such as added heating devices asused in conventional hot pressing or the use of die wall lubricatingsystems.

[0031] The transformation of the lubricant system from a solid to aliquid allows for higher press pressures without the formation ofmicro-cracks and delamination. During the pressing operation thelubricant is pressed to the die well, and helps to lower ejectionforces. Because there is less lubricant used in the green parts, thereis significantly less gas evolution during sintering, giving an operatorthe flexibility to increase loads, modify processing speeds and modifythe amount of carrier gas used during sintering. Specifically, forexample, applicant has seen as high as 70% reduction in gas evolutionusing a lubricant system made in accordance with the present invention(which made parts at a 0.3% by weight loading) as compared to aconventional lubricant such as ACRAWAX® (which required a loading of0.75% by weight). Applicant has also found that the lubricant system isremoved at a fairly steady rate over the entire de-lube heating cycle ofthe green part, whereas, with conventional lubricants meaningful removalgenerally only occurs at the higher temperatures of the cycle and atmuch greater rates as compared to the lubricant system of the presentinvention. Thus, the lubricant system of the present invention isespecially useful with metal powder systems containing nickel, for withsuch powder systems crack and blister formation is a problem duringelevated de-lubing temperatures wherein high lubricant removal rates areexperienced.

[0032] It will be appreciated that other additives may be used inconjunction with the lubricant system of the present invention. Forexample, sinter aids or small additions of alloying metals maybe addedto powder system. Also, it will be appreciated that the lubricant systemmaybe supplied in a concentrated form (master mix) and then let-down bythe end user. For example, a master mix comprising by weight 1% to 20%lubricant and 80% to 99% powder could be supplied, and the end usercould then add powder to bring the lubricant down to the 0.10% to 0.5%range.

[0033] It will also be appreciated that the lubricant system of thepresent invention may be used as a die wall lubricant and thus beapplied directly to the walls of the cavity wherein the green part isbeing formed.

[0034] The following examples serve to illustrate the invention, butthey are not intended to limit it thereto.

EXAMPLE I Preparation of Lubricant System

[0035] A mixture was prepared comprising by weight the followingcomponents:   10% lauric acid   10% stearic acid (impure comprisingabout 45% by weight palmitic acid)   50% ACRAWAX ® C-Lonza, Inc. 13.5%guanidine stearate 16.5% guanidine ethyl-hexonate

[0036] The lauric and stearic acid were preground in a Waring Blender toa particle size of about 100 mesh. All components are then double conemixed followed by melt mixing at 60° C.: The melt mixed product was thencryogenically ground to provide a particle size of about 10 to about 25μmicrons.

EXAMPLE II Preparation of Metal Mixes Using Lubricant System andComparative Material

[0037] A metal mix of the lubricant of Example I was prepared andprocessed as follows.

[0038] Nine parts by weight 409CS stainless steel powder to 1 partlubricant of Example 1 were hand mixed to remove agglomerates to providea master mix. The master mix was then let down in a double cone mixerwith additional stainless steel powder to provide a final mix comprising0.20% by weight lubricant. The mix provided the following properties.Lubricant In 409CS Stainless Steel Lubricant (by weight) 0.20% of(comparative) Example I 0.75% ACRAWAX ® C Property (Powder) ApparentDensity (g/cm³) 3.09 2.93 Flow Rate (seconds) 24.89 29.77 Property(Green) 30 tsi Press Density (g/cm³)* 5.700 5.692 Eject Force (lb.) 24501705 Green Strength (psi) 1454 1747 Property (sintered) Density(g/cm³)** 6.341 6.422 Carbon % 0.0267 0.0263 Property (Green) 40 tsiPress Density (g/cm³)* 6.085 6.153 Eject Force (lb.) 3850 2325 GreenStrength (psi) 2573 2407 Property (sintered)*** Density (g/cm³)** 6.6616.680 Carbon % 0.0278 0.0250 Property (Green) 48 tsi Press Density(g/cm³)* 6.284 6.363 Eject Force (lb.) 4750 2927 Green Strength (psi)3130 3260 Property (sintered)*** Density (g/cm³)** 6.883 6.837 Carbon %0.0294 0.0276

EXAMPLE III Additional Lubricant Systems

[0039] Additional lubricant systems made in accordance with the presentinvention A, B, C, D and E were prepared by mixing the componentsfollowed by batch melting. After melting the material was cooled andthen cryogenically ground to about 10 to about 25μ microns. The systemscomprised by weight the following components: Component A B C D E LauricAcid 10.0 5.0 15.0 10.0 Stearic Acid 10.0 15.0 15.0 10.0 (Impurecomprising about 45% by weight palmitic acid) ACRAWAX ® C 50.0 50.0 50.050.0 Lonza, Inc. Guanidine Stearate .45 12.3 .45 GuanidineEthyl-hexonate .55 17.7 .55 Micro-crystalline Wax M7381 11.9 12.0 10.012 Moore & Munger Marketing, Inc. Polyethylene Copolymer Wax 520 17.118.0 10.0 18 Clariant Corporation Butyl Stearate Wax (Liquid) 9.5Lithium Stearate¹ 50.0 Glycerol Monostearate-Emerest 240 9.5 CognisCorporation

EXAMPLE IV

[0040] Metal mixes were made with conventional lubricants and withlubricant systems made in accordance with the present invention(lubricant system A of Example III). The mixes provided the followingproperties which clearly show that the lubricant system made inaccordance with the present invention performs substantially as well orbetter than conventional lubricants used at much higher loadings.Lubricant System And Metal Powder 129 L U.S. Hoeganes Ampal AMB AmetekBronze¹ 129 L U.S. 85 HP² Hoeganes 2712 PMP³ Ampal AMB 304 L⁴ 1.00%Ametek 0.75%⁵ Bronze¹ 0.75% Acrawax 85 HP² 1.5% Acrawax 2712 PMP³Acrawax 304 L⁴ (Conventional) 0.30% A (Conventional) 0.25% A(Conventional) 0.30% A (Conventional) 0.25% A Properties (40 TSI) (40TSI) (60 TSI) (60 TSI) (30 TSI) (30 TSI) (60 TSI) (60 TSI) Green Density7.72 g/cc 7.76 g/cc 7.22 g/cc 7.35 g/cc 2.64 g/cc 2.67 g/cc 6.80 g/cc6.80 g/cc Green Strength 1804 psi 2887 psi 1874 psi 2034 997 1533 psi1447 psi 1223 psi Green Spring 0.26% 0.23% 0.28% 0.30% 0.17% 0.25% 0.35%0.41 Break-away Ejected 1533 lbf 1637 lbf 2584 lbf 3217 lbf 889 lbf 1528lbf 2682 lbf 3374 lbf Pressure Peak Stripping 2057 lbf 1952 lbf 2092 lbf2742 lbf 842 lbf 1408 lbf 2375 lbf 2867 lbf Pressure

[0041] While the invention has been explained in relation to itspreferred embodiments, it is to be understood that various modificationsthereof will become apparent to those skilled in the art upon readingthis specification. Therefore, it is to be understood that the inventiondisclosed herein is intended to cover such modifications as fall withinthe scope of the appended claims.

What is claimed is:
 1. A solid lubricant system for use in theproduction of metal powder compacted parts, said solid lubricant systembeing capable of forming a liquid phase upon application of pressure. 2.A solid lubricant system as set forth in claim 1 comprising a fattyacid.
 3. A solid lubricant system as set forth in claim 2 wherein saidfatty acid comprises lithium stearate.
 4. A solid lubricant system asset forth in claim 1 that displays a viscosity of from about 1000 toabout 6000 poise at a shear rate of 1000 ¹/second.
 5. A solid lubricantsystem as set forth in claim 1 for use as a die wall lubricant.
 6. Asolid lubricant system as set forth in claim 1 comprising a fatty acidmaterial and a guanidine material.
 7. A solid lubricant system as setforth in claim 1 comprising a guanidine material.
 8. A solid lubricantsystem as set forth in claim 1 comprising a synthetic wax and a fattyacid ester.
 9. A solid lubricant system as set forth in claim 1comprising an amide wax.
 10. A solid lubricant system as set forth inclaim 2 wherein said fatty acid comprises stearic acid.
 11. A solidlubricant system as set forth in claim 2 wherein said fatty acidcomprises lauric acid.
 12. A solid lubricant system as set forth inclaim 2 wherein said fatty acid comprises a mixture of stearic acid,palmitic acid and lauric acid.
 13. A solid lubricant system as set forthin claim 7 wherein said guanidine material comprises guanidine stearate14. A solid lubricant system as set forth in claim 7 wherein saidguanidine material comprises guanidine ethyl-neonate.
 15. A solidlubricant system as set forth in claim 7 wherein said guanidine materialcomprises a mixture of guanidine stearate and guanidine ethyl-hexonate.16. A composition for use in forming a pressed ceramic part comprising amixture of a ceramic powder and a solid lubricant system, at least aportion of said solid lubricant system converting to a liquid phase uponapplication of pressure to said composition.
 17. A composition as setforth in claim 16 wherein said lubricant system comprises a fatty acidmaterial and a guanidine material.
 18. A composition as set forth inclaim 16 wherein said lubricant system comprises a guanidine material.19. A composition as set forth in claim 16 wherein said lubricant systemcomprises a synthetic wax and a fatty acid ester.
 20. A composition asset forth in claim 18 wherein said guanidine material comprisesguanidine stearate.